1. Field of the Invention
The invention relates to a liquid crystal display device and method of manufacturing it, and more particularly, relates to a structure of a connection portion existing between a liquid crystal panel and a flexible substrate.
2. Description of the Related Art
There are various kinds of conventional liquid crystal display devices. In conventional liquid crystal display devices, by sealing a liquid crystal layer between two transparent substrates, and applying a voltage between the electrodes having fixed pattern formed inside of the substrates, it is possible to display various images.
In many cases, conventional liquid crystal display have a flexible substrate (referred to as an FPC hereinafter) to connect the liquid crystal display panel to control and drive circuits located outside of the liquid crystal display device. An example of the liquid crystal display device is described in the Utility Model (examined) Publication Hei 5-18789, (Japanese Utility Model (unexamined) Publication Hei 2-87473).
The connection portions have a predetermined pitch (namely the space as between adjacent wirings) adapted to the pitch of the external terminals, and the external terminals of the liquid crystal display device and the connection portions of the FPC are electrically connected to each other.
There are several methods to connect the FPC to the external terminals. One is to put the external terminals and other is to place an anisotropic conductive film between the external terminals and the connection portions, then adhering them by applying heat and pressure.
In the latter method (heat seal method), the anisotropic conductive film is formed on the surface of the connection portion of the flexible substrate, and conductive particles are arranged at equal interval which are made of a couple of xcexcm fine metal particles, then so plating the connection portion of the flexible substrate so as to contact to the external terminals. These heated components are pressed together so that the anisotropic conductive film is crushed and the external terminals and the connection portions are electrically connected to each other through the conductive particles. As mentioned above, the FPC is adhered to the liquid crystal panel easily by means of the anisotropic conductive film.
FIGS. 7, 8, 9, and 10 show an example of conventional technology using the heat seal method. FIG. 7 is a perspective view showing a connection area for connecting the FPC to the liquid crystal display panel, and FIG. 8 shows an enlarged cross-sectional view of an edge area of the liquid crystal display device. Typically transparent electrodes 3 are formed inside of a top substrate 1, and transparent electrodes 4 are also formed inside of a bottom substrate. 2. A liquid crystal layer 5 which is shown in FIG. 8 is sealed between top substrate 1 and bottom substrate 2, and a display area D is formed on the top substrate 1 as shown in FIG. 7. The bottom substrate extends outward beyond in comparison with the edge area of transparent substrate 1, and this extended portion forms an electrode lead-out area 2a. External terminals 3a, 4a are respectively connected to transparent electrodes 3, 4, which are transparent, formed in the shape of stripes and are arranged in parallel on the electrode lead-out area 2a.
On the other hand, in a resin sheet of a FPC 10, a plurality of conductive lines 12 are arranged almost in parallel, and the back side of the edge area of the conductive line 12 is exposed at a connection portion 10a of FPC 10. Connection portion 10a of FPC 10 is placed on the electrode lead-out area 2a as mentioned above and then heated and crimped. It is secured by the adhesive property of the anisotropic conductive film. As mentioned above, the external terminals 3a, 4a and the conductive lines 12 are electrically connected each other.
FIG. 9 depicts a printed circuit board 30 on which a liquid crystal drive circuit is mounted and connected to the liquid crystal display device through FPC 10. The back side of the conductive lines 12 are also exposed at a connection portion 10b on a circuit board 30, and the exposed area is also covered by the anisotropic conductive film. In the same way of above case, the connection portion 10b is secured to the printing circuit board 30, and the edge area of the conductive line 12 is conductivity connected to connection pads 31.
Conventional liquid crystal display devices have a number of disadvantages and problems as described below.
In the above mentioned conventional liquid crystal display device, when there is a transformation of the FPC, the stress caused by flexibility of the FPC may be applied to the conductive connection portion which is between the external terminals and connection portion. As a result wires may be cut, separated, or cause an interruption in the circuit. Therefore, reliability of such electrical connections of the conductive connection portion is reduced.
Particularly, in the above mentioned case, the thickness of the conductive layer on external terminals 3a, 4a is between 200 xc3x85 and 600 xc3x85. This is a result of a process which the liquid crystal display device is manufactured, and this is much thinner than the thickness between 10 xcexcm or 40 xcexcm which is the thickness of the connection pads 30 formed on a printing circuit board 31. Therefore, in the case of crimping the anisotropic conductive film, the space between the FPC and peripheral portion of the external terminals is narrow, so the space where extra adhesive or/and conductive particles can escape is also narrow. Consequently, the contact area with the conductive particles in the anisotropic conductive film becomes much smaller than the connection portion of the connection pads 31. As a result of this configuration, the resistance of the connection portion tends to increase, and the risk of a break in the circuit also increases. To improve this point, there may be a way to form the anisotropic conductive film of connection portion 10a so as to be thinner than connection portion 10b an so on. However, in this way, it is difficult to maintain the quality of the connection portion, and there is a problem that the cost for manufacturing FPC 10 increase, therefore, the problem can not be solved in the above way.
In addition, in so called heat seal method for connecting the FPC through the anisotropic conductive film, although it is an easier operation for connecting, there is a problem, however, that due to the temperature, the conductive particles float in the anisotropic conductive film. This results in the conductive particles being separated from upper surface of the external terminals, and the conductive connection between them are then easily broken.
Finally, in conventional devices, there is a tendency during the manufacturing process not to have a perfect alignment of arranging conductive line 12 on external terminal 4a. FIG. 10 is an exploded view of a single connection of line 12 on external terminal 4a. As shown therein, conductive line 12 is misaligned on external terminal 4a by an amount xcex4. This results in a smaller area of electrical contact, thus making a poorer contact having a high resistivity.
With the advent, in recent years, of large capacity size of liquid crystal display devices, and the number of the external terminals has increased accordingly, thus resulting in the pitch between terminals being very fine (which is a particular with requirement of small-sized devices). As such there has been an increase in defective products.
Therefore, it is an object of the present invention to overcome the aforementioned problems.
It is another object of the present invention to improve the reliability of electrical conductive connection between a liquid crystal panel and an FPC without increasing of cost for manufacturing.
It is a further object of this invention is to reduce the cost of liquid crystal display device and products using it, and to improve the durability of these products by improving the reliability of the electrically conductive connection portion and the registration rate of such products.
In accordance with an aspect of the present invention a liquid crystal display device is provided with a liquid crystal panel having a liquid crystal layer sealed between two substrates and an external terminal electrically connected to an electrode used for applying an electrical field to the liquid crystal layer. A flexible substrate includes a connection portion electrically connected to the external terminal, and a conductive layer having a fixed thickness is provided in a predetermined pattern between the external terminal and the connection portion.
In accordance with another aspect of the present invention a conductive layer having a fixed thickness is provided in a predetermined pattern between the external terminal, and the connection portion. Pressure supplied from outside is transmitted intensively to the portion between the external terminals and connection portions, and sufficient pressure is supplied to the portion between external terminal and connection portion. As a connection area formed by the thickness of conductive layer is attached to the connection portion, a contact area of a conductive adhesive provided between the external terminals and the connection portion is broaden. This results in maintaining a predetermined conductivity. As a result, the occurrence of conductive deterioration caused by stress or heat from outside is reduced. This is especially true in the case when the external terminals and the connection portion are electrically connected by sufficient pressure, when the flexible substrate is transformed along to the plane of by the conductive layer, and when contact pressure and the contact area is increased.
In this case, the conductive layer should preferably be formed on a surface of the external terminal by arranging a stiffened conductive paste in a predetermined pattern. In this way, it is easier to form the conductive layer.
The conductive paste should preferably be a conductive ink and the conductive layer should preferably be formed by printing. As such, the conductive layer can be formed in low cost.
Additionally, the conductive paste should preferably be formed using a carbon particle and a phenol resin as the main ingredient. In this case, electrical conductivity is sufficient, and making it possible to provide the conductive layer having certain conductivity without losing the conductive layer""s shape. This results since the conductive layer is stiffened by heating concurrently with connection.
Furthermore, the upper side of the conductive layer should preferably be formed having a toughened or irregularly shaped surface. As such, a conductive contact area is broaden and the adhesion force become stronger.
Additionally, an adhesive layer should preferably include conductive particles, which contact with the conductive layer, is formed between the external terminal and the connection portion. An anisotropic conductivity can be obtained, and as the conductive particles in the adhesive comes in contact with the conductive layer, large contact area and large compression force are obtained. In accordance with this arrangement, and durability of the contact area for the stress and heat from out side are improved.
Furthermore, the conductive layer should preferably consist of same material with a conductive connection portion for connecting the electrode of the liquid crystal panel and the external terminal. As such, the effect as mentioned above is obtained without adding other fabrication steps.
In a method of manufacturing a liquid crystal display devices of the invention, a conductive layer is formed in a predetermined pattern on a surface of an external terminal conductivity connected to an electrode for applying an electric field to a liquid crystal layer contained between two substrates of a liquid crystal panel, and a connection portion of a flexible substrate is electrically connected to the external terminal through the conductive layer. Especially, as the conductive layer is formed in a predetermined pattern on the external terminal at the same time of forming with the same material as a conductive connection portion for connecting the electrode and the external terminals conductivity, it is not necessary to add further fabrication steps. The liquid crystal display device equipped with this reliable connection portion between a liquid crystal panel and a flexible substrate can be obtained easily and at a low cost.
Additionally, it is preferable that the conductive layer is contacted once to both of the two substrate, then eliminating a part of one of the substrates which contacts to the conductive layer, so that upper surface of the conductive layer which remains on other the substrate is formed roughened or irregular surface. In this way, when a substrate is removed, upper portion of the conductive layer is separated with the substrate which is removed, therefore, it is possible to form the toughened surface of upper side of the conductive layer which remains. As a result, this arrangement of the adhesion force is improved and the contact area is increased.
Other objects and attainments together with a fuller understanding of the invention will become apparent and appreciated by referring to the following description and claims taken in conjunction with the accompanying drawings.